Add Path Sum III C#

Author: hellomrsun

August-LeetCoding-Challenge/08-Path-Sum-III/Path-Sum-III.cs

@@ -0,0 +1,42 @@
+public class Solution {
+    public int PathSum(TreeNode root, int sum) {
+        if (root == null) {
+            return 0;
+        }
+        var map = new Dictionary<int, int>();
+        map[0] = 1;
+        return Dfs(root, 0, sum, map); 
+    }
+    
+     private int Dfs(TreeNode curr, int sum, int target, Dictionary<int, int> map) {
+        if (curr == null) {
+            return 0;
+        }
+        // update the prefix sum by adding the current val
+        sum += curr.val;
+        // get the number of valid path, ended by the current node
+        int numPathToCurr = map.ContainsKey(sum-target) ? map[sum-target] : 0; 
+        // update the map with the current sum, so the map is good to be passed to the next recursion
+        map[sum] = (map.ContainsKey(sum) ? map[sum] : 0) + 1;
+        // add the 3 parts discussed in 8. together
+        int res = numPathToCurr + Dfs(curr.left, sum, target, map)
+                  + Dfs(curr.right, sum, target, map);
+       // restore the map, as the recursion goes from the bottom to the top
+        map[sum] -= 1;
+        return res;
+    }
+}
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public int val;
+ *     public TreeNode left;
+ *     public TreeNode right;
+ *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */

README.md

@@ -14,6 +14,7 @@ Solutions in various programming languages are provided. Enjoy it.
 5. [Add and Search Word Data structure design](https://github.com/AlgoStudyGroup/Leetcode/tree/master/August-LeetCoding-Challenge/05-Add-and-Search-Word-Data-structure-design)
 6. [Find All Duplicates in an Array](https://github.com/AlgoStudyGroup/Leetcode/tree/master/August-LeetCoding-Challenge/06-Find-All-Duplicates-in-an-Array)
 7. [Vertical Order Traversal of a Binary Tree](https://github.com/AlgoStudyGroup/Leetcode/tree/master/August-LeetCoding-Challenge/07-Vertical-Order-Traversal-of-a-Binary-Tree)
+8. [Path Sum III](https://github.com/AlgoStudyGroup/Leetcode/tree/master/August-LeetCoding-Challenge/08-Path-Sum-III)
 
 ## July LeetCoding Challenge
 Click [here](https://leetcode.com/explore/featured/card/july-leetcoding-challenge/) for problem descriptions.